Obama sends thoughts to Aust MH17 families

US President Barack Obama has sent his condolences to the families of Australian victims of the MH17 disaster.

Mr Obama has also paid tribute to AIDS researchers and activists who were bound for an international conference in Melbourne.

The president phoned Prime Minister Tony Abbott to discuss the downing of the Malaysia Airlines flight, which crashed in disputed territories in eastern Ukraine.

"The president expressed condolences on behalf of the American people to the Australian families who lost loved ones on board the flight," a statement from the White House said on Saturday.

Almost 300 passengers and crew died when MH17 was downed on Thursday, with 28 Australians and one American, among the casualties.

The talks come after Abbott and Australia's representative on the United Nations Security Council ramped up pressure on Russia to de-escalate its influence over separatist forces in eastern Ukraine.

"The two leaders discussed the importance of a prompt, full, unimpeded and transparent international investigation, and they stressed the need for immediate access by international investigators to the crash site," the statement said.

"The president underscored continuing U.S. support and co-operation with Australia as this issue unfolds, and the two leaders committed to remaining in close touch moving forward.

Mr Obama told a press conference that evidence indicated the jet was shot down by a surface-to-air-missile launched from an area controlled by Russian-back separatists in eastern Ukraine.

There were a number of AIDS researchers, activists and health workers headed for the 20th International AIDS Conference in Melbourne on the plane, and Mr Obama made special mention of them during his address.

"Onboard Malaysia Airlines flight 17 there were apparently nearly 100 researchers and advocates travelling to an international conference in Australia dedicated to combating AIDS/HIV," the president said.

"These were men and woman who have dedicated their own lives to saving the lives of others and were taken from us in a senseless act of violence.

"In this world today we shouldn't forget in the midst of conflict and killing there are people like these.

"People who are focused on what can be built, rather than what can be destroyed.

"People who are focused on how they can help people they have never met."